[9] RFR(S): 8079343: Crash in PhaseIdealLoop with "assert(!had_error) failed: bad dominance"

[Tobias Hartmann]

Hi,

please review the following patch.

https://bugs.openjdk.java.net/browse/JDK-8079343
http://cr.openjdk.java.net/~thartmann/8079343/webrev.00/

Problem:
The regression test for JDK-8078497 [1] fails with "assert(!had_error) failed: bad dominance" because during loop optimizations a node in the pre-loop has a control node in the main-loop. The test looks like this:


  byte[] src = src1;

  for (int i = ...) {

    // Copy 8 chars from src to dst

    ...

    // Prevent loop invariant code motion of char read.

    src = (src == src1) ? src2 : src1;

  }



The problem is that the superword optimization tries to vectorize the 8 load char / store char operations in the loop. In 'SuperWord::align_initial_loop_index' the base address of the memory operation we align the loop to ('align_to_ref_p.base()') is not loop invariant because 'src' changes in each iteration. However, we use it's value to correctly align the address if 'vw > ObjectAlignmentInBytes' (see line 2344). This is wrong because the address changes in each iteration. It also causes follow up problems in 'PhaseIdealLoop::verify_dominance' because nodes in the pre-loop have a control in the main-loop.


Solution:
I agreed with Vladimir K. that we should not try to vectorize such edge cases. I added a check to the constructor of SWPointer to bail out in case the base address is loop variant.

Testing:
- Regression test
- JPRT (together with fix for JDK-8078497)

Thanks,
Tobias

[1] https://bugs.openjdk.java.net/browse/JDK-8078497